1. Field of Invention
This invention relates to the field of extruders and injection machines utilizing a rotatable screw within a heated barrel, and more specifically to a screw structure which improves mixing of the injection material while that material is within the barrel.
2. Description of the Related Art
Extruders and injection molding machines are well known. One of the common problems in the use of such a machine is the differences in the material properties of the material due to temperature differentials between points close to the inner surface of the barrel wall and points farther away. Because the barrel is heated, a temperature differential commonly exists across the material between these points. It is preferable that this temperature differential be minimized in order to avoid over heating the injection material or otherwise damaging its properties. One way to minimize such differential is to mix the resinous material while it is in the barrel.
Another common problem concerns molding of products utilizing resinous materials of different colors. When resinous materials of two or more colors are to be molded, any incomplete mixing of such resinous materials could mar the appearance of the finished molded product.
For these and other reasons, better mixing of resinous materials within extruder barrels has been desired within the industry.
Various methods of providing such mixing are known in the art. For example, in U.S. Pat. No. 3,006,029 to Saxton, an extruder mixing screw is disclosed having primary channels and minor channels. In addition, U.S. Pat. No. 5,035,509 to Kruder discloses a screw with a zig zag barrier for mixing within the mixing section of the screw. U.S. Pat. No. 5,071,256 to Smith et al. discloses a screw with barrier near the outlet end of the screw. Finally, U.S. Pat. No. 4,752,136 to Colby discloses a plasticating apparatus and rotating screw having at least two paths of different depths near an outlet end of the screw.
Although some of these screws are effective for their purpose, further improvements are desirable.
The present invention contemplates a new and improved plasticating apparatus and method which overcomes the foregoing difficulties and others while providing better and more advantageous overall results.